1. Field of the Invention
This invention relates to a tubular membrane separation apparatus of the self-cleaning type.
2. Description of the Prior Art
Ultrafiltration and reverse osmosis apparatuses are known wherein a feedwater is pressurized and concentrated on the inner side of a tubular membrane to yield a purified water on the outer side of said tubular membrane.
Such apparatuses are each comprised of a single or a plurality of modules, which are units of tubular membrane, and whereas an ultrafiltration apparatus incorporates comparatively coarse membranes for separatory removal of the macromolecules contained in feedwater, a reverse-osmosis apparatus comprises comparatively dense membranes so as to separate small-sized particulate contaminants such as ions.
The reverse-osmosis technique is capable of separating and concentrating non-ionizable contaminants which cannot be separated by electrodyalysis, which involves the use of a similar membrane, and has therefore been commonly utilized for the separation of inorganic substances as well as for the separation and concentration of organic materials.
Compared with other types of reverse-osmosis equipment, the reverse-osmosis apparatus employing tubular membranes, in particular, is advantageous in that it does not require any elaborate pretreatment and withstands rugged conditions of use. However, as it is the case with other reverse-osmosis equipment, the fouling matter accumulates of necessity as the process of concentration proceeds, so that the permeation performance of the membranes is gradually reduced. This problem is shared by ultrafiltration equipment. The fouling matter mentioned above comprises the suspended matter, organic matter and microorganisms contained in the feedwater as well as the insoluble salts, metal hydrozides and so on which are deposited on the internal surface of the membrane with the progress of concentration and, usually cannot be readily removed by flushing or by washing with a chemical solution.
Once the fouling matter begins to accumulate on the inner surface of the membrane, the accumulation and growth of such fouling matter proceeds at an accelerated rate with the initial deposits acting as nuclei. Therefore, to prevent a reduction in permeation performance of the equipment, the internal surface of the membrane must be kept clean at all times while the equipment is operated.
To solve this problem, S. Leob et al proposed in 1966 a mechanical cleaning method which comprises passing cleaning balls made of an elastic material such as sponge rubber through the tubular membrane units to remove the fouling matter. This method has thence been improved and such improvements include the following.
A. A method of cleaning the internal surface of a tubular membrane which comprises sending cleaning balls along with a cleaning solution from the feedwater inlet side while the equipment is resting idle.
B. A method of cleaning the interval surface of a tubular membrane which comprises entrapping cleaning balls between the two screens disposed adjacent the feedwater inlet and the concentrate outlet, respectively, of the membraneous separation apparatus and causing the balls to ply between the screens as the direction of flow of the water is reversed.
C. A method of cleaning the internal surface of a membrane which comprises sending cleaning balls from a ball feeding element disposed adjacent the feedwater inlet of the apparatus to clean the internal surface of the membrane and withdrawing the used balls from a ball-takeout element disposed adjacent the concentrated-water outlet of the apparatus.
The tubular membrane separation apparatuses embodying those methods, however, are invariably inadequate in the case of control and in efficiency. More particularly, the known apparatuses have the following disadvantages.
The first method A is inefficient in that, to effect the necessary cleaning, the operation of the apparatus must be suspended, thus precluding the benefit of a continuous operation.
The second method B not only involves additional first costs, e.g. the costs of additional piping and switch valve means for reversing the flow, but a complexity of switching operation and, hence, problems related to process control. Moreover, because the switching operation is of necessity accompanied by interruptions of flow and pressure fluctuations, the efficiency of water-treatment is sacrified. Furthermore, the reversing of flow is not feasible in the tubular membrane separation apparatus comprising a Christmas-tree arrangement of modules.
The third method C is the most desirable of all in that since the apparatus can be cleaned during its operation, the design efficiency can be fully maintained and the initial cost of the apparatus is comparatively low. Nonetheless, because independent devices must respectively be provided for feeding and withdrawing cleaning balls, the cleaning operation cannot be carried out in a closed circuit, particularly when frequent cleanings at short intervals are required, such that the method has problems related to operation control.